Three esters of N-acyl phenyl isoserine, taxol, taxotere and cephalomannine have been found to possess significant properties as antitumor agents. This application describes a process for the preparation of N-acyl, N-sulfonyl and N-phosphoryl substituted isoserine esters, in general and to a semi-synthesis for the preparation of taxane derivatives such as taxol, taxotere and other biologically active derivatives involving the use of metal alkoxides and xcex2-lactams, in particular.
The taxane family of terpenes, of which taxol is a member, has attracted considerable interest in both the biological and chemical arts. Taxol is a promising cancer chemotherapeutic agent with a broad spectrum of antileukemic and tumor-inhibiting activity. Taxol has the following structure: 
wherein Ac is acetyl. Because of this promising activity, taxol is currently undergoing clinical trials in both France and the United States.
The supply of taxol for these clinical trials is presently being provided by the bark from Taxus brevifollia (Western Yew). However, taxol is found only in minute quantities in the bark of these slow growing evergreens, causing considerable concern that the limited supply of taxol will not meet the demand. Consequently, chemists in recent years have expended their energies in trying to find a viable synthetic route for the preparation of taxol. So far, the results have not been entirely satisfactory.
One synthetic route that has been proposed is directed to the synthesis of the tetracyclic taxane nucleus from commodity chemicals. A synthesis of the taxol congener taxusin has been reported by Holton, et al. in JACS 110, 6558 (1988). Despite the progress made in this approach, the final total synthesis of taxol is, nevertheless, likely to be a multi-step, tedious, and costly process.
A semi-synthetic approach to the preparation of taxol has been described by Greene, et al. in JACS 110, 5917 (1988), and involves the use of a congener of taxol, 10-deacetyl baccatin III which has the structure of formula II shown below: 
10-deacetyl baccatin III is more readily available than taxol since it can be obtained from the needles of Taxus baccata. According to the method of Greene et al., 10-deacetyl baccatin III is converted to taxol by attachment of the C-10 acetyl group and by attachment of the C-13 xcex2-amido ester side chain through the esterification of the C-13 alcohol with a xcex2-amido carboxylic acid unit. Although this approach requires relatively few steps, the synthesis of the xcex2-amido carboxylic acid unit is a multi-step process which proceeds in low yield, and the coupling reaction is tedious and also proceeds in low yield. However, this coupling reaction is a key step which is required in every contemplated synthesis of taxol or biologically active derivative of taxol, since it has been shown by Wani, et al. in JACS 93, 2325 (1971) that the presence of the xcex2-amido ester side chain at C13 is required for anti-tumor activity.
More recently, it has been reported in Colin et al. U.S. Pat. No. 4,814,470 that taxanes corresponding to the following formula III, have an activity significantly greater than that of taxol (I). 
Rxe2x80x2 represents hydrogen or acetyl and one of Rxe2x80x3 and Rxe2x80x3xe2x80x2 represents hydroxy and the other represents tert-butoxy-carbonylamino and their stereoisomeric forms, and mixtures thereof.
According to Colin et al., U.S. Pat. No. 4,418,470, the products of general formula (III) are obtained by the action of the sodium salt of tert-butyl N-chlorocarbamate on a product of general formula: 
in which Rxe2x80x2 denotes an acetyl or 2,2,2-trichloroethoxycarbonyl radical, followed by the replacement of the 2,2,2-trichloroethoxycarbonyl group or groups by hydrogen. It is reported by Denis et al. in U.S. Pat. No. 4,924,011, however, that this process leads to a mixture of isomers which has to be separated and, as a result, not all the baccatin III or 10-deactylbaccatin III employed for the preparation of the product of general formula (IV) can be converted to a product of general formula (III).
In an effort to improve upon the Colin et al. process, Denis et al. disclose a different process for preparing derivatives of baccatin III or of 10-deactylbaccatin III of general formula 
in which Rxe2x80x2 denotes hydrogen or acetyl wherein an acid of general formula: 
in which R1 is a hydroxy-protecting group, is condensed with a taxane derivative of general formula: 
in which R2 is an acetyl hydroxy-protecting group and R3 is a hydroxy-protecting group, and the protecting groups R1, R3 and, where appropriate, R2 are then replaced by hydrogen. However, this method employs relatively harsh conditions, proceeds with poor conversion, and provides less than optimal yields.
A major difficulty remaining in the synthesis of taxol and other potential anti-tumor agents is the lack of a readily available method for easy attachment, to the C-13 oxygen, of the chemical unit which provides the xcex2-amido ester side chain. Development of such a process for its attachment in high yield would facilitate the synthesis of taxol as well as related anti-tumor agents having a modified set of nuclear substituents or a modified C-13 side chain. This need has been fulfilled by the discovery of a new, efficient process for attachment, to the C-13 oxygen, of the chemical unit which provides the xcex2-amido ester side chain.
Another major difficulty encountered in the synthesis of taxol is that known processes for the attachment of the xcex2-amido ester side chain at C-13 are generally not sufficiently diastereoselective. Therefore, the side chain precursor must be prepared in optically active form to obtain the desired diastereomer during attachment. The process of this invention, however, is highly diastereoselective, thus permitting the use of a racemic mixture of side chain precursor, eliminating the need for the expensive, time-consuming process of separating the precursor into its respective enantiomeric forms. The reaction additionally proceeds at a faster rate than previous processes, thus permitting the use of less side-chain precursor than has been required by such previous processes.
Among the objects of the present invention, therefore, is the provision of a process for the preparation of N-acyl, N-sulfonyl and N-phosphoryl esters of isoserine; the provision of a side chain precursor for the synthesis of taxane derivatives; the provision of a process for the attachment of the side chain precursor in relatively high yield to provide an intermediate which is readily converted to the desired taxane derivative; and the provision of such a process which is highly diastereoselective.
In accordance with the present invention, a process is provided for the preparation of isoserine esters having the formula 
comprising reacting a xcex2-lactam with an alkoxide, the xcex2-lactam having the formula 
and the alkoxide having the formula
MOCE1E2E3
wherein
R1 is xe2x80x94OR6, xe2x80x94SR7, or xe2x80x94NR8R9;
R2 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;
R3 and R4 are independently hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, acyl or heterosubstituted alkyl, alkenyl, alkynyl, aryl or heteroaryl, provided, however, that R3 and R4 are not both acyl;
R5 is xe2x80x94COR10, xe2x80x94COOR10, xe2x80x94COSR10, xe2x80x94CONR8R10, or xe2x80x94SO2R11,
R6 is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, hydroxy protecting group, or a functional group which increases the water solubility of the taxane derivative,
R7 is alkyl, alkenyl, alkynyl, aryl, heteroaryl, or sulfhydryl protecting group,
R8 is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, or heterosubstituted alkyl, alkenyl, alkynyl, aryl or heteroaryl;
R9 is an amino protecting group;
R10 is alkyl, alkenyl, alkynyl, aryl, heteroaryl, or heterosubstituted alkyl, alkenyl alkynyl, aryl or heteroaryl;
R11 is alkyl, alkenyl, alkynyl, aryl, heteroaryl, xe2x80x94OR10, or xe2x80x94NR8R14;
R14 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;
E1, E2 and E3 are independently hydrogen, hydrocarbon or cyclic, provided, at least one of E1, E2 and E3 is other than hydrogen; and
M comprises ammonium or is a metal.
In accordance with another aspect of the present invention, the alkoxide and xcex2-lactam are selected so as to provide a process for preparing taxol, taxotere and other biologically active taxane derivatives having the following structural formula: 
wherein
R1-R14 are as previously defined,
R15 is hydrogen or together with R16 forms an oxo,
R16 is hydrogen, xe2x80x94OCOR29, hydroxy, or protected hydroxy, or together with R15 forms an oxo;
R17 is hydrogen or together with R18 forms an oxo,
R18 is hydrogen, hydroxy, protected hydroxy, acyloxy, or together with R17 forms an oxo;
R19 is hydrogen or together with R20 forms an oxo,
R20 is hydrogen, halogen, protected hydroxy, xe2x80x94OR28, or together with R19 forms an oxo;
R21 is hydrogen or together with R22 forms an oxo,
R22 is hydrogen, hydroxy, protected hydroxy, acyloxy, together with R21 forms an oxo, or together with R23 and the carbon atoms to which they are attached form an oxetane ring;
R23 is hydrogen, together with R24 forms an oxo, or together with R22 and the carbon atoms to which they are attached form an oxetane ring;
R24 is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, cyano, hydroxy, xe2x80x94OCOR30, or together with R23 forms an oxo, oxirane or methylene;
R25 is hydrogen, hydroxy, or xe2x80x94OCOR31;
R26 is hydrogen or taken together with R25 forms an oxo;
R27 is hydrogen, hydroxy, protected hydroxy;
R28 is hydrogen, acyl, hydroxy protecting group or a functional group which increases the solubility of the taxane derivative; and
R29, R30, and R31 are independently hydrogen, alkyl, alkenyl, alkynyl, monocyclic aryl or monocyclic heteroaryl.
Briefly, therefore, the taxane derivatives are prepared by reacting a xcex2-lactam (2) with an alkoxide having the bi-, tri- or tetracyclic taxane nucleus to form a xcex2-amido ester intermediate. The intermediate is then converted to the taxane derivative. xcex2-lactam (2) has the general formula: 
wherein R1-R5 are as previously defined. The alkoxide preferably has the tricyclic taxane nucleus corresponding to the general formula: 
wherein M and R15-R27 are as previously defined. Most preferably, the alkoxide has the tetracyclic taxane nucleus corresponding to alkoxide (3) wherein R22 and R23 together form an oxetane ring.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.
As used herein xe2x80x9cArxe2x80x9d means aryl; xe2x80x9cPhxe2x80x9d means phenyl; xe2x80x9cAcxe2x80x9d means acetyl; xe2x80x9cEtxe2x80x9d means ethyl; xe2x80x9cRxe2x80x9d means alkyl unless otherwise defined; xe2x80x9cBuxe2x80x9d means butyl; xe2x80x9cPrxe2x80x9d means propyl; xe2x80x9cTESxe2x80x9d means triethylsilyl; xe2x80x9cTMSxe2x80x9d means trimethylsilyl; xe2x80x9cTPAPxe2x80x9d means tetrapropylammonium perruthenate; xe2x80x9cDMAPxe2x80x9d means p-dimethylamino pyridine; xe2x80x9cDMFxe2x80x9d means dimethylformamide; xe2x80x9cLDAxe2x80x9d means lithium diisopropylamide; xe2x80x9cLAHxe2x80x9d means lithium aluminum hydride; xe2x80x9cRed-Alxe2x80x9d means sodium bis(2-methoxyethoxy) aluminum hydride; xe2x80x9cAIBNxe2x80x9d means azo-(bis)-isobutyronitrile xe2x80x9c10-DABxe2x80x9d means 10-desacetylbaccatin III; protected hydroxy means xe2x80x94OR wherein R is a hydroxy protecting group; sulfhydryl protecting groupxe2x80x9d includes, but is not limited to, hemithioacetals such as 1-ethoxyethyl and methoxymethyl, thioesters, or thiocarbonates; xe2x80x9camine protecting groupxe2x80x9d includes, but is not limited to, carbamates, for example, 2,2,2-trichloroethylcarbamate or tertbutylcarbamate; and xe2x80x9chydroxy protecting groupxe2x80x9d includes, but is not limited to, ethers such as methyl, t-butyl, benzyl, p-methoxybenzyl, p-nitrobenzyl, allyl, trityl, methoxymethyl, methoxyethoxy-methyl, ethoxyethyl, tetrahydropyranyl, tetrahydrothio-pyranyl, and trialkylsilyl ethers such as trimethylsilyl ether, triethylsilyl ether, dimethylarylsilyl ether, triisopropylsilyl ether and t-butyldimethylsilyl ether; esters such as benzoyl, acetyl, phenylacetyl, formyl, mono-, di-, and trihaloacetyl such as chloroacetyl, dichloroacetyl, trichloroacetyl, trifluoroacetyl; and carbonates including but not limited to alkyl carbonates having from one to six carbon atoms such as methyl, ethyl, n-propyl, isopropyl, n-butyl, t-butyl; isobutyl, and n-pentyl; alkyl carbonates having from one to six carbon atoms and substituted with one or more halogen atoms such as 2,2,2-trichloroethoxymethyl and 2,2,2-trichloroethyl; alkenyl carbonates having from two to six carbon atoms such as vinyl and allyl; cycloalkyl carbonates have from three to six carbon atoms such as cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl; and phenyl or benzyl carbonates optionally substituted on the ring with one or more C1-6 alkoxy, or nitro. Other hydroxyl, sulfhydryl and amine protecting groups may be found in xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d by T. W. Greene, John Wiley and Sons, 1981.
The alkyl groups described herein, either alone or with the various substituents defined hereinabove are preferably lower alkyl containing from one to six carbon atoms in the principal chain and up to 15 carbon atoms. They may be straight or branched chain and include methyl, ethyl, propyl, isopropyl, butyl, hexyl and the like.
The alkenyl groups described herein, either alone or with the various substituents defined hereinabove are preferably lower alkenyl containing from two to six carbon atoms in the principal chain and up to 15 carbon atoms. They may be straight or branched chain and include ethenyl, propenyl, isopropenyl, butenyl, isobutenyl, hexenyl, and the like.
The alkynyl groups described herein, either alone or with the various substituents defined hereinabove are preferably lower alkynyl containing from two to six carbon atoms in the principal chain and up to 15 carbon atoms. They may be straight or branched chain and include ethynyl, propynyl, butynyl, isobutynyl, hexynyl, and the like.
The aryl moieties described herein, either alone or with various substituents, contain from 6 to 15 carbon atoms and include phenyl. Substituents include alkanoxy, protected hydroxy, halogen, alkyl, aryl, alkenyl, acyl, acyloxy, nitro, amino, amido, etc. Phenyl is the more preferred aryl.
The heteroaryl moieties described herein, either alone or with various substituents, contain from 5 to 15 atoms and include, furyl, thienyl, pyridyl and the like. Substituents include alkanoxy, protected hydroxy, halogen, alkyl, aryl, alkenyl, acyl, acyloxy, nitro, amino, and amido.
The acyloxy groups described herein contain alkyl, alkenyl, alkynyl, aryl or heteroaryl groups.
The heterosubstituents of the heterosubstituted alkyl, alkenyl, alkynyl, aryl, and heteroaryl moieties described herein, contain nitrogen, oxygen, sulfur, halogens and/or one to six carbons, and include lower alkoxy such as methoxy, ethoxy, butoxy, halogen such as chloro or fluoro, and nitro.
The present invention is directed to a process for preparing substituted isoserine esters, in general, and taxol, taxotere and other taxane derivatives which are biologically active using xcex2-lactam (2), the structure of which is depicted hereinbelow: 
wherein R1, R2, R3, R4 and R5 are as previously defined.
In accordance with the present invention, R5 of xcex2-lactam (2) is preferably xe2x80x94COR10 or xe2x80x94COOR10 with R10 being lower alkyl, aryl, heteroaryl (such as furyl or thienyl), or substituted phenyl, and most preferably phenyl, methyl, ethyl, tert-butyl, or 
wherein X is Cl, Br, F, CH3Oxe2x80x94, or NO2xe2x80x94. Preferably R2 and R4 are hydrogen or lower alkyl. R3 is preferably aryl, most preferably, naphthyl, phenyl, 
wherein X is as previously defined, Me is methyl and Ph is phenyl. Preferably, R1 is selected from xe2x80x94OR6, xe2x80x94SR7 or xe2x80x94NR8R9 wherein R6, R7 and R9, are hydroxy, sulfhydryl, and amine protecting groups, respectively, and R8 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl. Most preferably, R1 is xe2x80x94OR6 wherein R6 is triethylsilyl (xe2x80x9cTESxe2x80x9d), 1-ethoxyethyl (xe2x80x9cEExe2x80x9d) or 2,2,2-trichloroethoxymethyl.
As noted above, R1 of xcex2-lactam (2) may be xe2x80x94OR6 with R6 being alkyl, acyl, ethoxyethyl (xe2x80x9cEExe2x80x9d), triethylsilyl (xe2x80x9cTESxe2x80x9d), 2,2,2-trichloroethoxymethyl, or other hydroxyl protecting group such as acetals and ethers, i.e., methoxymethyl (xe2x80x9cMOMxe2x80x9d), benzyloxymethyl; esters, such as acetates; carbonates, such as methyl carbonates; and alkyl and aryl silyl such as triethylsilyl, trimethylsilyl, dimethyl-t-butylsilyl, dimethylarylsilyl, dimethyl-heteroarylsilyl, and triisopropylsilyl, and the like. A variety of protecting groups for the hydroxyl group and the synthesis thereof may be found in xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d by T. W. Greene, John Wiley and Sons, 1981. The hydroxyl protecting group selected should be easily removed under conditions that are sufficiently mild, e.g., in 48% HF, acetonitrile, pyridine, or 0.5% HCl/water/ethanol, and/or zinc, acetic acid so as not to disturb the ester linkage or other substituents of the taxol intermediate. However, R6 is preferably triethylsilyl, 1-ethoxyethyl or 2,2,2-trichloroethoxymethyl, and most preferably triethylsilyl.
Since xcex2-lactam (2) has several asymmetric carbons, it is known to those skilled in the art that the compounds of the present invention having asymmetric carbon atoms may exist in diastereomeric, racemic, or optically active forms. All of these forms are contemplated within the scope of this invention. More specifically, the present invention includes enantiomers, diastereomers, racemic mixtures, and other mixtures thereof.
xcex2-lactam (2) can be prepared from readily available materials, as is illustrated in schemes A and B below: 
reagents: (a) triethylamine, CH2Cl2, 25xc2x0 C., 18 h; (b) 4 equiv ceric ammonium nitrate, CH3CN, xe2x88x9210xc2x0 C., 10 min; (c) KOH, THF, H2O, 0xc2x0 C., 30 min; (d) ethyl vinyl ether, THF, toluene sulfonic acid (cat.), 0xc2x0 C., 1.5 h; (e) n-butyllithium, ether, xe2x88x9278xc2x0 C., 10 min; benzoyl chloride, xe2x88x9278xc2x0 C., 1 h; (f) lithium diisopropyl amide, THF xe2x88x9278xc2x0 C. to xe2x88x9250xc2x0 C.; (g) lithium hexamethyldisilazide, THF xe2x88x9278xc2x0 C. to 0xc2x0 C.; (h) THF, xe2x88x9278xc2x0 C. to 25xc2x0 C., 12 h.
The starting materials are readily available. In scheme A, xcex1-acetoxy acetyl chloride is prepared from glycolic acid, and, in the presence of a tertiary amine, it cyclocondenses with imines prepared from aldehydes and p-methoxyaniline to give 1-p-methoxyphenyl-3-acyloxy-4-arylazetidin-2-ones. The p-methoxyphenyl group can be readily removed through oxidation with ceric ammonium nitrate, and the acyloxy group can be hydrolyzed under standard conditions familiar to those experienced in the art to provide 3-hydroxy-4-arylazetidin-2-ones. The 3-hydroxyl group is protected with 1-ethoxyethyl, but may be protected with variety of standard protecting groups such as the triethylsilyl group or other trialkyl (or aryl) silyl groups. In Scheme B, ethyl-xcex1-triethylsilyloxyacetate is readily prepared from glycolic acid.
The racemic xcex2-lactams may be resolved into the pure enantiomers prior to protection by recrystallization of the corresponding 2-methoxy-2-(trifluoromethyl) phenylacetic esters. However, the reaction described hereinbelow in which the xcex2-amido ester side chain is attached has the advantage of being highly diastereoselective, thus permitting the use of a racemic mixture of side chain precursor.
The 3-(1-ethoxyethoxy)-4-phenylazetidin-2-one of scheme A and the 3-(1-triethylsilyloxy)-4-phenylazetidin-2-one of scheme B can be converted to xcex2-lactam (2), by treatment with a base, preferably n-butyllithium, and an acyl chloride, alkylchloroformate, sulfonyl chloride, phosphinyl chloride or phosphoryl chloride at xe2x88x9278xc2x0 C. or below.
The process of the present invention is particularly useful for the esterification of mono- or polycyclic alkoxides represented by the formula 
in which E1, E2 and the carbon to which they are attached define a carbocyclic and/or heterocyclic skeleton which may be mono- or polycyclic and E3 is hydrogen or hydrocarbon, preferably lower alkyl. Most preferably, the carbocyclic and/or heterocyclic skeleton comprises about 6 to 20 atoms and the hetero atoms are oxygen. The cyclic skeleton may be hydrocarbon and/or heterosubstituted with heterosubstituents including, for example, esters, ethers, amines, alcohols, protected alcohols, carbonyl groups, halogens, oxygen, substituted oxygen or substituted nitrogen.
When the alkoxides have the bi-, tri- or tetracyclic taxane nucleus, the process of the present invention may advantageously be used to prepare taxane derivatives, many of which have been found to have significant biological activity. As used herein, an alkoxide having the bicyclic taxane nucleus has the carbocyclic skeleton corresponding to rings A and B of metal alkoxide (3): 
wherein M and R15-R27 are as previously defined. An alkoxide having the tricyclic taxane nucleus has the carbocyclic skeleton corresponding to rings A, B and C of metal alkoxide (3). An alkoxide having the tetracyclic taxane nucleus has carbocyclic rings A, B and C of metal alkoxide (3) and the oxetane ring defined by R22, R23, and the carbons to which they are attached.
Substituent, M, of alkoxide 3 is a metal or comprises ammonium. The metal may be a Group IA, IIA, transition (including lanthanides and actinides), IIB, IIIA IVA, VA, or VIA metal (CAS version). The ammonium comprising substituent is preferably tetraalkylammonium and the alkyl component of the tetraalkylammonium substituent is preferably C1-C10 alkyl such as methyl or butyl.
Alkoxides (3) are prepared by reacting an alcohol having two to four rings of the taxane nucleus and a C-13 hydroxyl group with an organometallic compound, zinc chloride dimethoxyethane complex, anhydrous cadmium chloride, or a tetraalkylammonium halide such as tetrabutylammonium chloride in a suitable solvent. Preferably, the alcohol is a derivative of baccatin III or 10-deacetyl baccatin III having the structure: 
wherein R16-R20, R24, and R26 are as previously defined. More preferably, R16 is protected hydroxy or xe2x80x94OCOR29, R19 is hydrogen, R20 is hydrogen, halogen or protected hydroxy, R24 is acetoxy, and R26 is benzoyloxy Most preferably, the alcohol is a protected baccatin III, in particular, 7-O-triethylsilyl baccatin III (which can be obtained as described by Greene, et al. in JACS 110, 5917 (1988) or by other routes) or 7,10-bis-O-triethylsilyl baccatin III.
As reported in Greene et al., 10-deacetyl baccatin III is converted to 7-O-triethylsilyl-10-deacetyl baccatin III according to the following reaction scheme: 
Under what is reported to be carefully optimized conditions, 10-deacetyl baccatin III is reacted with 20 equivalents of (C2H5)3SiCl at 23xc2x0 C. under an argon atmosphere for 20 hours in the presence of 50 ml of pyridine/mmol of 10-deacetyl baccatin III to provide 7-triethylsilyl-10-deacetyl baccatin III (6a) as a reaction product in 84-86% yield after purification. The reaction product is then acetylated with 5 equivalents of CH3COCl and 25 mL of pyridine/mmol of (6a) at 0xc2x0 C. under an argon atmosphere for 48 hours to provide 86% yield of 7-O-triethylsilyl baccatin III (6b). Greene, et al. in JACS 110, 5917 at 5918 (1988).
Alternatively, 7-triethylsilyl-10-deacetyl baccatin III (6a) can be protected at C-10 oxygen with an acid labile hydroxyl protecting group. For example, treatment of (6a) with n-butyllithium in THF followed by triethylsilyl chloride (1.1 mol equiv.) at 0xc2x0 C. gives 7,10-bis-O-triethylsilyl baccatin III (6c) in 95% yield. Also, (6a) can be converted to 7-O-triethylsilyl-10-(1-ethoxyethyl) baccatin III (6d) in 90% yield by treatment with excess ethyl vinyl ether and a catalytic amount of methane sulfonic acid. These preparations are illustrated in the reaction scheme below. 
7-O-triethylsilyl baccatin III (6b), 7,10-bis-O-triethylsilyl baccatin III (6c), or 7-O-triethylsilyl-10-(1-ethoxyethyl) baccatin III (6d) is reacted with an organometallic compound such as n-butyllithium in a solvent such as tetrahydrofuran (THF), to form the metal alkoxide 13-O-lithium-7-O-triethylsilyl baccatin III (7b) 13-O-lithium-7,10-bis-O-triethylsilyl baccatin III (7c), or 13-O-lithium-7-O-triethylsilyl-10-(1-ethoxyethyl) baccatin III (7d) as shown in the following reaction scheme: 
As illustrated in the following reaction scheme, a suitable metal alkoxide of the present invention such as 13-O-lithium-7-O-triethylsilyl baccatin III derivative (7b, 7c, or 7d) reacts with a xcex2-lactam of the present invention to provide an intermediate (8b, 8c, or 8d) in which the C-7 hydroxyl group is protected with a triethylsilyl or 1-ethoxyethyl group. 
Intermediate compound (8b) readily converts to taxol when R1 is xe2x80x94OR6, R2 and R3 are hydrogen, R4 is phenyl, R5 is benzoyl and R6 is a hydroxy protecting group such as triethylsilyl. Intermediate compound (8c) readily converts to taxotere when R1 is xe2x80x94OR6, R2 and R3 are hydrogen, R4 is phenyl, R5 is tertbutoxycarbonyl and R6 is a hydroxy protecting group such as triethylsilyl. Intermediate compound (8d) readily converts to 10-deacetyl taxol when R1 is xe2x80x94OR6, R2 and R3 are hydrogen, R4 is phenyl, R5 is benzoyl, and R6 is a hydroxy protecting group such as triethylsilyl. Intermediate compounds (8b, 8c and 8d) may be converted to the indicated compounds by hydrolyzing the triethylsilyl and 1-ethoxyethyl groups under mild conditions so as not to disturb the ester linkage or the taxane derivative substituents. 
Both the conversion of the alcohol to the metal alkoxide and the ultimate synthesis of the taxol can take place in the same reaction vessel. Preferably, the xcex2-lactam is added to the reaction vessel after formation therein of the metal alkoxide.
The organometallic compound n-butyllithium is preferably used to convert the alcohol to the corresponding metal alkoxide, but other sources of metallic substituent such as lithium diisopropyl amide, other lithium or magnesium amides, ethylmagnesium bromide, methylmagnesium bromide, other organolithium compounds, other organomagnesium compounds, organosodium, organotitanium, organozirconium, organozinc, organocadmium or organopotassium or the corresponding amides may also be used. Organometallic compounds are readily available, or may be prepared by available methods including reduction of organic halides with metal. Lower alkyl halides are preferred. For example, butyl bromide can be reacted with lithium metal in diethyl ether to give a solution of n-butyllithium in the following manner: 
Alternatively, the lithium alkoxide may be induced to undergo exchange with metal halides to form alkoxides of aluminum, boron, cerium, calcium, zirconium or zinc.
Although THF is the preferred solvent for the reaction mixture, other ethereal solvents, such as dimethoxyethane, or aromatic solvents may also be suitable. Certain solvents, including some halogenated solvents and some straight-chain hydrocarbons in which the reactants are too poorly soluble, are not suitable. Other solvents are not appropriate for other reasons. For example, esters are not appropriate for use with certain organometallic compounds such as n-butyllithium due to incompatibility therewith.
Although the reaction scheme disclosed herein is directed to the synthesis of certain taxol derivatives, it can be used with modifications in either the xcex2-lactam or the tetracyclic metal alkoxide. Therefore, alkoxides other than 13-O-lithium-7-O-triethylsilyl baccatin III may be used to form a taxol or other taxanes according to the method of this invention. The xcex2-lactam and the tetracyclic alkoxide can be derived from natural or unnatural sources, to prepare other synthetic taxols, taxol derivatives, 10-deacetyltaxols, and the enantiomers and diastereomers thereof contemplated within the present invention.
The process of the invention also has the important advantage of being highly diastereoselective. Therefore racemic mixtures of the side chain precursors may be used. Substantial cost savings may be realized because there is no need to resolve racemic xcex2-lactams into their pure enantiomers. Additional cost savings may be realized because less side chain precursor, e.g., 60-70% less, is required relative to prior processes.
The water solubility of compounds of formula (3) may be improved by modification of the C2xe2x80x2 and/or C7 substituents. For instance, water solubility may be increased if R1 is xe2x80x94OR6 and R20 is xe2x80x94OR28, and R6 and R28 are independently hydrogen or xe2x80x94COGCOR1 wherein
G is ethylene, propylene, xe2x80x94CHxe2x95x90CHxe2x80x94, 1,2-cyclohexane, or 1,2-phenylene,
R1=OH base, NR2R3, OR3, SR3, OCH2CONR4R5, OH
R2=hydrogen, methyl
R3=(CH2)nNR6R7; (CH2)nN⊕R6R7R8X63 .
n=1 to 3
R4=hydrogen, lower alkyl containing 1 to 4 carbons
R5=hydrogen, lower alkyl containing 1 to 4 carbons, benzyl, hydroxyethyl, CH2CO2H, dimethylaminoethyl
R6R7=lower alkyl containing 1 or 2 carbons, benzyl or R6 and
R7 together with the nitrogen atom of NR6R7 form the following rings 
R8=lower alkyl containing 1 or 2 carbons, benzyl
Xxe2x8ax96=halide
base=NH3, (HOC2H4)3N, N(CH3)3, CH3N(C2H4OH)2, NH2(CH2)6NH2, N-methylglucamine, NaOH, KOH.
The preparation of compounds in which X1 or X2 is xe2x80x94COGCOR1 is set forth in Hangwitz U.S. Pat. No. 4,942,184 which is incorporated herein by reference.
Alternatively, water solubility may be increased when R1 is xe2x80x94OR6 and R6 is a radical having the formula xe2x80x94COCXxe2x95x90CHX or xe2x80x94COXxe2x80x94CHXxe2x80x94CHXxe2x80x94SO2Oxe2x80x94M wherein X is hydrogen, alkyl or aryl and M is hydrogen, alkaline metal or an ammonio group as described in Kingston et al., U.S. Pat. No. 5,059,699 (incorporated herein by reference).
Taxanes having alternative C9 keto substituent may be prepared by selective reduction to yield the corresponding C9 xcex2-hydroxy derivative. The reducing agent is preferably a borohydride and, most preferably, tetrabutylammoniumborohydride (Bu4NBH4) or triacetoxyborohydride.
As illustrated in Reaction Scheme 1, the reaction of baccatin III with Bu4NBH4 in methylene chloride yields 9-desoxo-9xcex2-hydroxybaccatin III 5. After the C7 hydroxy group is protected with the triethylsilyl protecting group, for example, a suitable side chain may be attached to 7-protected-9xcex2-hydroxy derivative 6 as elsewhere described herein. Removal of the remaining protecting groups thus yields 9xcex2-hydroxy-desoxo taxol or other 9xcex2-hydroxytetracylic taxane having a C13 side chain. 
Alternatively, the C13 hydroxy group of 7-protected-9xcex2-hydroxy derivative 6 may be protected with trimethylsilyl or other protecting group which can be selectively removed relative to the C7 hydroxy protecting group as illustrated in Reaction Scheme 2, to enable further selective manipulation of the 1 various substituents of the taxane. For example, reaction of 7,13-protected-9xcex2-hydroxy derivative 7 with KH causes the acetate group to migrate from C10 to C9 and the hydroxy group to migrate from C9 to C10, thereby yielding 10-desacetyl derivative 8. Protection of the C10 hydroxy group of 10-desacetyl derivative 8 with triethylsilyl yields derivative 9. Selective removal of the C13 hydroxy protecting group from derivative 9 yields derivative 10 to which a suitable side chain may be attached as described above. 
As shown in Reaction Scheme 3, 10-oxo derivative 11 can be provided by oxidation of 10-desacetyl derivative 8. Thereafter, the C13 hydroxy protecting group can be selectively removed followed by attachment of a side chain as described above to yield 9-acetoxy-10-oxo-taxol or other 9-acetoxy-10-oxotetracylic taxanes having a C13 side chain. Alternatively, the C9 acetate group can be selectively removed by reduction of 10-oxo derivative 11 with a reducing agent such as samarium diiodide to yield 9-desoxo-10-oxo derivative 12 from which the C13 hydroxy protecting group can be selectively removed followed by attachment of a side chain as described above to yield 9-desoxo-10-oxo-taxol or other 9-desoxo-10-oxotetracylic taxanes having a C13 side chain. 
Reaction Scheme 4 illustrates a series of reactions in which 10-DAB is used as the starting material. Reduction of 10-DAB yields pentaol 13, the C7 and C10 hydroxyl groups of which can be selectively protected with the triethylsilyl or another protecting group to produce triol 14. A C13 side chain can be attached to triol 14 as described above or, alternatively, after further modification of the tetracylic substituents. 
Taxanes having C9 and/or C10 acyloxy substituents other than acetate can be prepared using 10-DAB as a starting material as illustrated in Reaction Scheme 5. Reaction of 10-DAB with triethylsilyl chloride in pyridine yields 7-protected 10-DAB 15. The C10 hydroxy substituent of 7-protected 10-DAB 15 may then be readily acylated with any standard acylating agent to yield derivative 16 having a new C10 acyloxy substituent. Selective reduction of the C9 keto substituent of derivative 16 yields 9xcex2-hydroxy derivative 17 to which a C13 side chain may be attached. Alternatively, the C10 and C9 groups can be caused to migrate as set forth in Reaction Scheme 2, above. 
Taxanes having alternative C2 and/or C4 esters can be prepared using baccatin III and 10-DAB as starting materials. The C2 and/or C4 esters of baccatin III and 10-DAB can be selectively reduced to the corresponding alcohol(s) using reducing agents such as LAH or Red-Al, and new esters can thereafter be substituted using standard acylating agents such as anhydrides and acid chlorides in combination with an amine such as pyridine, triethylamine, DMAP, or diisopropyl ethyl amine. Alternatively, the C2 and/or C4 alcohols may be converted to new C2 and/or C4 esters through formation of the corresponding alkoxide by treatment of the alcohol with a suitable base such as LDA followed by an acylating agent such as an acid chloride.
Baccatin III and 10-DAB analogs having different substituents at C2 and/or C4 can be prepared as set forth in Reaction Schemes 6-10. To simplify the description, 10-DAB is used as the starting material. It should be understood, however, that baccatin III derivatives or analogs may be produced using the same series of reactions (except for the protection of the CIO hydroxy group) by simply replacing 10-DAB with baccatin III as the starting material. 9-desoxo derivatives of the baccatin III and 10-DAB analogs having different substituents at C2 and/or C4 can then be prepared by reducing the C9 keto substituent of these analogs and carrying out the other reactions described above.
In Reaction Scheme 6, protected 10-DAB 3 is converted to the triol 18 with lithium aluminum hydride. Triol 18 is then converted to the corresponding C2 ester using Cl2CO in pyridine followed by a nucleophilic agent (e.g., Grignard reagents of alkyllithium reagents). 
Alternatively, deprotonation of triol 18 with LDA followed by introduction of an acid chloride selectively gives the C4 ester. For example, when acetyl chloride was used, triol 18 was converted to 1,2 diol 4 as set forth in Reaction Scheme 7.
As set forth in Reaction Scheme 9, other C4 substituents can be provided by reacting carbonate 19 with an acid chloride and a tertiary amine to yield carbonate 22 which is then reacted with alkyllithiums or Grignard reagents to provide 10-DAB derivatives having new substituents at C2.
Alternatively, baccatin III may be used as a starting material and reacted as shown in Reaction Scheme 10. After being protected at C7 and C13, baccatin III is reduced with LAH to produce 1,2,4,10 tetraol 24. Tetraol 24 is converted to carbonate 25 using Cl2CO and pyridine, and carbonate 25 is acylated at C10 with an acid chloride and pyridine to produce carbonate 26 (as shown) or with acetic anhydride and pyridine (not shown). Acetylation of carbonate 26 under vigorous standard conditions provides carbonate 27 which is then reacted with alkyl lithiums to provide the baccatin III derivatives having new substituents at C2 and C10. 
9-desoxo-10-desacetoxy derivatives of baccatin III and 9-desoxo-10-desoxy derivatives of 10-DAB may be prepared by reacting baccatin III or 10-DAB (or their derivatives) with samarium diiodide and thereafter reducing the C9 keto substituent as otherwise described herein. Reaction between the tetracyclic taxane having a C10 leaving group and samarium diiodide may be carried out at 0xc2x0 C. in a solvent such as tetrahydrofuran. Advantageously, the samarium diiodide selectively abstracts the C10 leaving group; C13 side chains and other substituents on the tetracyclic nucleus remain undisturbed.
C7 dihydro and other C7 substituted taxanes can be prepared as set forth in Reaction Schemes 11 and 12.
As shown in Reaction Scheme 12, Baccatin III may be converted into 7-fluoro baccatin III by treatment with diethylaminosulfur trifluoride (DAST) at room temperature in THF solution. Other baccatin derivatives with a free C7 hydroxyl group behave similarly. Alternatively, 7-chloro baccatin III can be prepared by treatment of baccatin III with methane sulfonyl chloride and triethylamine in methylene chloride solution containing an excess of triethylamine hydrochloride.
A wide variety of tricyclic taxanes are naturally occurring, and through manipulations analogous to those described herein, an appropriate side chain can be attached to the C13 oxygen of these substances. Alternatively, as shown in Reaction Scheme 13, 7-O-triethylsilyl baccatin III can be converted to a tricyclic taxane through the action of trimethyloxonium tetrafluoroborate in methylene chloride solution. The product diol then reacts with lead tetraacetate to provide the corresponding C4 ketone. This ketone can be reduced to the alcohol with a hydride reducing agent such as sodium borohydride and subsequent acetylation produces the C4, C5 diacetate. 
The following examples are provided to more fully illustrate the invention.